1. Field of the Invention
This invention relates to new and useful improvements in novolak resin binders, to the novel process for applying these binders to discrete, inert solid particles, to the novel composition produced thereby, and to novel processes which employ these compositions.
2. The Prior Art
Resin binders have been employed heretofore in the preparation of consolidated materials by bonding discrete inert solid particles such as sand, abrasive grit, wood chips, and a suitable binder.
The shell molding process for the production of sand mold sections for the casting of metals is well-known in the prior art. While there are many variations of this process, the process essentially comprises depositing a combination of sand and potentially thermosetting solid resin against a heated pattern such that the resin melts and cures to form a rigid shell mold section for use in the casting of metals. The resin coated sand used in the process provides free-flowing sand grains, such having a solid, non-tacky coating of resin.
The production of a shell or mold involves two basic steps; the invest and the cure step. In the first step, the resin-coated sand is dumped onto or blown against the pattern (invested) until the shell is thick enough to hold metal in a given application. The time required for this operation is called the "invest time". The faster the shell forms, the shorter the invest time can be. The rate at which the shell forms is known as the "build-up rate". The faster the build-up rate, the shorter the invest time. If faster build-up rates can be attained, faster production cycles can be run. In the second step, the uncured resin-coated sand is dumped or dropped away from the shell on bonded coated particles of sand and the resulting shell is cured. After the shell is cured, it is removed from the hot metal pattern and is ready for use. Therefore, reducing the cure time required also can accelerate the production of shell cores and molds.
Phenolic resins are known to be particularly useful in the shell molding process. For shell molding, two-step phenol-formaldehyde resins (also known as novolaks) which are potentially thermosetting are employed. Thermoplastic phenol-formaldehyde novolak resins can be made potentially thermosetting by incorporating a curing agent such as hexamethylenetetramine. (Useful examples of potentially thermosetting phenolic resin-coated sands are disclosed in U.S. Pat. Nos. 2,706,163 and 2,888,418).
Foundry cores and foundry molds can be formed by the shell process. Foundry cores can also be formed in other processes which can employ one-step phenol-formaldehyde resins (also known as resoles), but such are not generally useful in the shell process.
Resin-coated sands exhibiting significantly increased build-up rate and/or cure rate in the shell process would be desirable in that such sands would permit shell cores and molds to be made at a faster rate. In this regard, the prior art shows that some effort has been made heretofore. For example, U.S. Pat. No. 3,471,443 discloses a potentially thermosetting phenol-formaldehyde novolak resin containing an aniline salt which is reported to be fast curing. While this composition may be effective, aniline salts are not desirable materials for inclusion in resin-coated sands because they are toxic and have an unpleasant pungent odor. Further, U.S. Pat. No. 3,838,095 discloses a novolak resin which includes urea to increase both the invest rate and the cure rate.
A desirable binder composition for a resin-coated sand would be a two-step novolak resin which exhibits both fast invest times and core times.